1. Technical Field
The present invention relates to a method of producing a retardation plate. The present invention also relates to a retardation plate obtained by the production method, a polarizing plate with a retardation plate including the retardation plate, a liquid crystal panel including the retardation plate, and a liquid crystal display apparatus including the liquid crystal panel.
2. Background Art
A liquid crystal display apparatus, etc. involve such a problem that a display screen is colored due to a retardation caused by a liquid crystal cell. In order to solve the problem, a retardation plate is used. The retardation plate is generally used while being attached to a polarizing plate.
A retardation plate to be used in a liquid crystal display apparatus, etc. desirably exhibits its function sufficiently with respect to incident light in the entire visible light region. In particular, the retardation plate desirably has so-called reverse dispersion characteristics in which an in-plane direction retardation increases with an increase in a wavelength. As a retardation plate having reverse dispersion characteristics, a laminated retardation plate in which retardation plates having different wavelength dispersion characteristics are laminated so that slow axes are perpendicular to each other (for example, see Patent Document 1).
In the laminated retardation plate, retardation plates to be laminated are attached to each other with a pressure-sensitive adhesive. Therefore, optical properties may decrease due to the presence of a pressure-sensitive adhesive layer. Further, when the retardation plates are attached to each other with a pressures-sensitive adhesive, an axis shift of the retardation plates to be laminated occurs, with the result that retardation plates having reverse dispersion characteristics may not be obtained.
In order to prevent the axis shift of a laminated retardation plate, a method of producing a laminated retardation plate having reverse dispersion characteristics by co-extruding melt of a resin having positive alignment birefringence and melt of a resin having negative alignment birefringence to form a laminate, and stretching the laminate in the same direction (for example, see Patent Document 2).
In the case of producing a laminated retardation plate by the co-extrusion and stretching, respective layers are stretched simultaneously in a uniaxial direction to express a retardation. Generally, in order to express a retardation, stretching is performed in the vicinity of a glass transition temperature (Tg) of a resin contained in each layer. In the case where Tg's of resins of the layers greatly differs from each other, a retardation is difficult to be expressed when the stretching temperature is adjusted to a higher Tg, and retardation unevenness occurs when the stretching temperature is adjusted to a lower Tg. In such cases, it is difficult to set an appropriate stretching temperature, and it is necessary to limit a combination of resins to be used so that the difference in glass transition temperature (Tg) of the resins contained in the respective layers is small.
In the case where the thickness of a laminate increases so as to express a desired retardation, it is necessary to increase a stretching temperature so as to stretch the laminate uniformly. Even in such a case, there arises problems that a retardation is not expressed and retardation unevenness occurs.    Patent Document 1: JP 04-343303 A    Patent Document 2: JP 2002-107542 A